There are a number of applications which require an amplifier/comparator that has a fast slew rate in one direction from a positive to a negative voltage or in the opposite direction from a negative to a positive voltage. Typically, amplifiers of this type are combined as two separate amplifier/comparator stages, the first one providing a positive transition and the second providing a negative transition. The outputs of the two amplifiers are then combined with a circuit such as a set/reset flip-flop, with the output waveform transitions having slew rates determined by either the first or second amplifier/comparator stage.
One previous type of amplifier/comparator stage for providing a fast slew rate in one direction only utilizes a full Class A type comparator. In this device, a differential MOS device is provided having two separate legs, each for receiving a separate polarity of a differential input voltage, with a common current source providing current to both legs. The lower ends of each of the legs are coupled through a current mirror configuration, with the output of one leg driving the gate of a switching element. Depending upon the state of the differential input, one leg is always drawing current. This current is utilized during switching between two differential states to charge the associated node capacitance of the current mirror configuration. Therefore, in either state the current must be sufficient to charge this capacitor. When this differential configuration is utilized in a one way amplifier/comparator, the differential configuration drives an output switch element. The switch element has an active state with the switch element conducting and an inactive state with the switch element non-conducting. The current for the differential configuration driving the switch element is the same for either the active or the inactive state, resulting in a large power requirement for the device when the switch element is not actually conducting, i.e., when it is not in the fast slewing mode. This presents a disadvantage to the design of low power, high speed devices. There therefore exists a need for a low power fast one way amplifier/comparator.